Measurements made in a borehole may be used to identify the composition and distribution of material that surrounds the measurement device. However, the accuracy of the identification may depend on how precisely the tool can be located in the borehole. For example, the position of the tool is sometimes determined using a two-part process. In the first part, an ultrasonic transducer, also known to those of ordinary skill in the art as a mud transducer, is used to generate acoustic pulses that traverse separate paths, to determine ultrasonic velocity in the drilling fluid (or “mud”), using the time difference between the pulses upon traversing their respective path lengths. In the second part, the drilling fluid velocity defined by the mud transducer measurement can be used to determine the tool standoff in the borehole, using the time delay between pulse transmission and reception for a pitch-catch transducer.
However, the traditional ultrasonic tool may lack the capability to accurately determine drilling fluid density, which is assumed to be relatively constant during logging. For example, due to drilling fluid gravity and/or environmental changes, the drilling fluid density can vary, and the assumption is invalid. These variations can therefore lead to systematic standoff measurement errors.